Is a flat top actually flat?

Bob-Dubery

Think of a steel-string acoustic guitar, with a hole in the middle and the strings terminating at the bridge (either in anchor holes cut into the bridge or by means of bridge pins). Should one expect the top of such a guitar to be actually flat? It seems to me that there must always be a tendency for the bridge to rotate, and so the top should be slightly concave in front of the bridge and slightly convex behind.

What say you wise folks of GFSA?

Jack-Flash-Jr

Think of it this way:

Squonk

Jack Flash Jr wrote Think of it this way:

PeteM

Relevance?

AlanRatcliffe

X-rated Bob wrote Should one expect the top of such a guitar to be actually flat?

Pretty much, but it's a lot of pressure and most are not perfectly flat - especially with older guitars. A little bellying up is acceptable, but if the bridge starts leaning, it will need attention because that means the top is collapsing.

Bob-Dubery

OK.... the question is partly on behalf of a friend who isn't a member here (and is real luddite and thus probably doesn't have much inclination get involved in a web forum ?). He has a Taylor. When he got it it had (we think) 80:20 12s on it. He looked at the Taylor web site and they say they fit all their guitars with "Elixir nanoweb lights". So he went out and got a set of "Elixir nanoweb lights", but phosphor bronze. He liked the sound. Then he noticed that there was a little bit of a ripple behind the bridge, a little bit of caving in in front. He says that this hadn't been therefore - and he's a meticulous, observant sort.

Now in the meantime I've done some googling. Some guys say that a bit of a bulge is OK, but a ripple is not.

Then I start looking at strings and tension. Elixir only publish tensions for 80/20s. But D'addario publish tensions for all their sets. Their 80/20 12s are the same gauges and nearly the same tensions as the Elixirs (D'addario provide figures to 1 decimal, Elixir seem to round up to the nearest integer).

So, to keep it on the shortish side
Elixir 80/20 12s - total of 159 pounds
Elixir 80/20 13s - total of 183 pounds
D'addario 80/12s - total of 159.4 pounds
D'addario PB 12s - total of 163.6 pounds

So going from 12s to 13s you add about 24 pounds.
Going from 80/20 to PB but keeping the gauge the same you add about 4.2 pounds. Which sounds like not very much (and certainly not as much as going from lights to mediums) but seems to have caused this slight ripple in the top.

But then I suppose if it's built JUST strongly enough to take the 80/20 lights and they keep their margins slim then 4.2 pounds will be enough.

makepeace

this is interesting. surely if one got the maths and material selection right, by pre-stressing the top you could build an acoustic without bracing?

Keira-WitherKay

Strange how this came up on forum cos just yesterday i had a visit from a luthier who brought thru 3 of his flat tops for me to see and play and the topic that flat tops are not totally flat came up . And i was told the way to see it clearly is when top end guitars are under stage lights it's easier to see by the reflections or rather how it reflects light , so yes i too heard from a luthier who makes flat tops in the high price range about 40k a pop give or take that it is so and an interesting fact is that luthiers need to find that fine balance of a strong enough top but if it's too strong it's less responsive . Anyway i won't hi jack this thread but soon i'll post something about these guitar i played , the name they sell under is ' jacobs guitars ' and oh boy was i impressed and i don't even have a passion for flat tops as much as many here but i'll post later about em .

Jack-Flash-Jr

PeteM wrote Relevance?

What may appear flat from a distance is not when you take a closer look.

Bob-Dubery

makepeace wrote this is interesting. surely if one got the maths and material selection right, by pre-stressing the top you could build an acoustic without bracing?

yes and no.

You could build a guitar to withstand a certain strain without bracing. You might have to have a carved arch top, or you might have to cut the top 3/4 of an inch thick. What it would sound like is another matter.

As other folks are saying in this thread, the luthier is trying to balance strength and tone. That's why there are different bracing patterns for guitars. That's why classical guitars are braced quite differently from most steel string flat tops. That's why some luthiers have shaped braces that get that shape by having wood removed from them. A guy I know has a Martin D18 that had a fairly major overhaul a while back. As part of this the braces were shaved to a shape similar to that on older Martin guitars. He likes the sound now, but he can't put medium strings (13s) on it because the guitar won't take that sort of stress any more.

Instruments get built to a specification, to a requirement. A lot of old violins have been modified to deal with newer kinds of strings and because concert pitch has raised over the years. If nothing else changed, the change in the pitching of the A note (now 440 hz, but lower in days gone by) would put more strain on those instruments.

What might get close to what you envision is the carbon fibre guitars that some companies make today. I've never got to play one, but what I hear is that they're pretty good.

Anyhoo... how much difference should that 4 pounds make? I knew that PB strings exert more tension than 80/20s of the same gauge, but 4 pounds doesn't sound like that a big deal to me. Though, of course, if the guitar is built to withstand 120, 124 is going to cause problems.

PeteM

Jack Flash Jr wrote
PeteM wrote Relevance?
What may appear flat from a distance is not when you take a closer look.

I was curious to know why Bob was interested. He explained why in his second post.

Gearhead

At the risk of sounding like Brainy Smurf: all materials have a tendency to deform under stress. We should realise that the only way to exert a force as a solid body is to first deform. This means that regardless of the material, a flattop that is flat when not strung will be not flat under string tension.
The next question is, good or bad and how much? I reckon a flattop sounds like it is supposed to if the top is as free as possible, in other words the flatter the better. Arches make for stiffer construction so they stop the motion; the shorter the radius the stiffer the top. Braces also make the top stiffer.
On my wish-I-had-time-to-do list is a flattop without braces or tailpiece, but with a lever under the bridge, not unlike Breedlove. I reckon if you counter the string force with carbon rope running underneath the top to the end block, and you counter the moment between bridge and rope by a second rope running from the end of the lever to the neck heel, you can get away with next to no bracing and you can flatten the top by adjusting rope tension.

vic

The simple answer is YES. For all practical purposes a flat top's top is flat. What is the deviation from perfect flatness when strung and brought to pitch....maybe 0.9 % ? ? And presumably the belly will also become "less flat"under similar conditions.